Electronic displays are ubiquitous in today's world. For example, mobile devices such as smartphones and tablet computers commonly use a liquid crystal display (LCD) or an organic light emitting diode (OLED) display. LCDs and OLED displays are both examples of flat panel displays, and are also used in desktop monitors, TVs, and automotive and aircraft displays.
Many color displays, including many LCD and OLED displays, spatially synthesize color. In other words, each pixel is composed of three sub-pixels that provide a different color. For instance, each pixel can have a red, green, or blue sub-pixel, or a cyan, magenta, or yellow sub-pixel. The color of the pixel, as perceived by a viewer, depends upon the relative proportion of light from each of the three sub-pixels.
Color information for a display is commonly encoded as an RGB signal, whereby the signal is composed of a value for each of the red, green, and blue components of a pixel color for each signal in each frame. A so-called gamma correction is used to convert the signal into an intensity or voltage to correct for inherent non-linearity in a display, such that the intended color is reproduced by the display.
In the field of color science when applied to information display, colors are often specified by their chromaticity, which is an objective specification of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and saturation (s). Color spaces (e.g., the 1931 CIE XYZ color space or the CIELUV color space) are commonly used to quantify chromaticity. For instance, when expressed as a coordinate in a color space, a pixel's hue is the angular component of the coordinate relative to the display's white point, and its saturation is the radial component. Once color coordinates are specified in one color space, it is possible to transform them into other color spaces.
Humans perceive color in response to signals from photoreceptor cells called cone cells, or simply cones. Cones are present throughout the central and peripheral retina, being most densely packed in the fovea centralis, a 0.3 mm diameter rod-free area in the central macula. Moving away from the fovea centralis, cones reduce in number towards the periphery of the retina. There are about six to seven million cones in a human eye.
Humans normally have three types of cones, each having a response curve peaking at a different wavelength in the visible light spectrum. FIG. 1A shows the response curves for each cone type. Here, the horizontal axis shows light wavelength (in nm) and the vertical scale shows the responsivity. In this plot, the curves have been scaled so that the area under each cone is equal, and adds to 10 on a linear scale. The first type of cone responds the most to light of long wavelengths, peaking at about 560 nm, and is designated L for long. The spectral response curve for L cones is shown as curve A. The second type responds the most to light of medium-wavelength, peaking at 530 nm, and is abbreviated M for medium. This response curve is curve B in FIG. 1A. The third type responds the most to short-wavelength light, peaking at 420 nm, and is designated S for short, shown as curve C. The three types have typical peak wavelengths near 564-580 nm, 534-545 nm, and 420-440 nm, respectively; the peak and absorption spectrum varies among individuals. The difference in the signals received from the three cone types allows the brain to perceive a continuous range of colors, through the opponent process of color vision.
In general, the relative number of each cone type can vary. Whereas S-cones usually represent between 5-7% of total cones, the ratio of L and M cones can vary widely among individuals, from as low as 5% L/95% M to as high as 95% L/5% M. The ratio of L and M cones also can vary, on average, between members of difference races, with Asians believed to average close to 50/50 L:M and Caucasians believed to average close to 63% L cones (see, for example, U.S. Pat. No. 8,951,729). Color vision disorders also impact the proportion of L and M cones; protanopes have 0% L cones and deuteranopes have 0% M cones. Referring to FIG. 1B, cones are generally arranged in a mosaic on the retina. In this example, L and M cones are distributed in approximately equal numbers, with fewer S cones. Accordingly, when viewing an image on an electronic display, the response of the human eye to a particular pixel will depend on the color of that pixel and where on the retina the pixel is imaged.